core: set cmdContext sessionId on CommandBase ctr

Set session ID on CommandContext at CommandBase constructor
when not available. Needed for SEAT mechanism since session ID
is available only on parameters upon command re-instantiation
(when moving between task handlers).

Change-Id: I99bfbf7af491ec01e2dcc7d18b27c84d881c241f
Bug-Url: https://bugzilla.redhat.com/1159632
Bug-Url: https://bugzilla.redhat.com/1148495
Signed-off-by: Daniel Erez <derez@redhat.com>